Are you trying to recover a Point-in-Time? If so are you trying the last snapshot? You could attempt one snapshot before if possible.
In case that doesn't work either:
On OS level could you do under <SID>adm a HDB stop and then check HDB info if anything is still running for the nameserver? Also check whether the lock files are released. If the lock files are not released, please delete them manually.
In case this still doesn't work, you can also backup and delete the nameserver.ini to ensure it uses the system default found under /sapmnt/<SID>/global/hdb/custom/config
Worst case you could restore your latest full backup.
Regards,
Nick